This invention is an improvement in the apparatus and process disclosed in U.S. Pat. No. 4,798,711 entitled PROCESSES FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES AND HYDROGEN SULFIDE FROM GAS STREAMS, issued Jan. 17, 1989, Lewis G. Neal, et al. inventors, and assigned to the same assignee as the present invention. This prior art process is referred to in the art as the NOXSO Process. The NOXSO Process is illustrated in FIG. 1 wherein there is shown a flue gas stream, 12 containing both nitrogen oxides or NO.sub.x (NO and NO.sub.2 OR N.sub.2 O.sub.4) and sulfur oxides or SO.sub.x (SO.sub.2 and SO.sub.3) from, for example, a coal-fired or oil-fired power plant (not shown) which flue gas stream is passed through a fluid bed adsorber 14 containing suitable sorbent particles or beads, such as, for example, those disclosed in U.S. Pat. No. 4,755,499 entitled SORBENT FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES AND HYDROGEN SULFIDE FROM GAS STREAMS, issued Jul. 5, 1988, Lewis G. Neal, et al., inventors, and assigned to the same assignee as the present invention. Adsorber 14 has a fluidizing grid 15. The sulfur oxides and nitrogen oxides are adsorbed on the surfaces of the sorbent particles and removed from the flue gas stream. The two above-identified patents are hereby incorporated herein by reference as if fully reproduced herein.
The saturated sorbent particles 16, i.e. sorbent particles having adsorbed the NO.sub.x and SO.sub.x from the flue gas, is subsequently transported to a staged, fluid bed heater 18 wherein the sorbent particles temperature is raised above 532.degree. C. (1000.degree. F.) using high temperature air 20 supplied by air heater 22 into which air heater a stream of ambient air 24 and a suitable fuel stream 26, e.g., natural gas, enter. The sorbed NO.sub.x is removed or stripped from the sorbent particles and carried away in the hot gas stream which passes through cyclone 28 and via stream 30 is mixed with the power plant combustion air stream (not shown).
The hot sorbent particles with the NO.sub.x removed therefrom is transferred from the sorbent heater 18 into a moving bed regenerator 32 via line 34. In the moving bed regenerator 32, the sorbent particles are contacted with a suitable regenerant gas stream 36. The regenerant gas 36 reacts with the SO.sub.x adsorbed by the sorbent particles to produce elemental sulfur. Off-gas stream 38 containing elemental sulfur is transported into a sulfur condenser and mist eliminator 45 wherein a steam stream 42, water stream 44 and elemental sulfur stream 46 are produced. A stream 40 from the sulfur condenser and mist eliminator 45 is returned to regenerator 32.
The regenerated sorbent particles, i.e. sorbent particles with the SO.sub.x and NO.sub.x removed, is transported via stream 48 past valve 50 to a staged, fluid bed sorbent cooler 52, where it is contacted with atmospheric air supplied via line 54 to reduce its temperature to about 120.degree. C. (250.degree. F.). The heated atmospheric air 56 subsequently is transported to gas heater 22 where its temperature is increased well above 532.degree. C. (1000.degree. F.) for use as the heated medium in fluid bed heater 18.
Cooled regenerated sorbent particles via line 58 is transported via line 58 by air in line 54 to a pneumatic lift line 60 and into a cyclone separator 62 via stream 64. Cyclone separator 62 separates stream 64 into a stream of air 66 and a stream of regenerated sorbent particles 68. Regenerated sorbent stream 68 enters the fluid bed adsorber 14. The discharge gas from adsorber 14 exits via line 70.
In brief summary, it will be understood that the SO.sub.x and NO.sub.x are removed from the flue gas stream 12 by the fluid bed adsorber 14 to produce a stream of NO.sub.x and SO.sub.x free flue gas 70 and thereafter the fluid bed heater 18, moving bed regenerator 32 and fluid bed sorbent cooler 52 regenerate the sorbent particles which adsorbed the SO.sub.x and NO.sub.x in the fluid bed adsorber 14 whereafter the regenerated sorbent particles are transported via line 58 to the pneumatic lift line 60 for return through the cyclone separator 62 to the fluid bed absorber 14 where the NOXSO Process is repeated.
The above-described NOXSO Process has been found to be highly efficient in the removal of SO.sub.x and NO.sub.x from flue gas and since the sorbent particles or beads utilized are relatively large, approximately 10 mesh (2,000 microns) to 20 mesh (840 microns) (note incorporated U.S. Pat. No. 4,755,499, Col. 8, line 8), the process has the advantage that the sorbent particle or bead size is large relative to fly ash particles typically found in flue gas and therefore such sorbent beads or particles are easily distinguished from the fly ash and easily separated therefrom. A further advantage is that due to the relatively large size (approximately 10 to 20 mesh) the sorbent particles or beads have a relatively large mass which means they have a relatively high terminal velocity which substantially precludes the sorbent particles or beads from leaving the fluid or fluidized state and escaping from the fluid bed heater 18 and fluid bed cooler 52 with the exiting gases and being lost; the terminal velocity is the velocity at which the sorbent particles or beads in a fluid bed cease being in the fluid or fluidized state and escape from the fluid bed and become subject to entrainment into gas exiting the fluid bed and loss. However, the above-described NOXSO Process has some disadvantage in that the relatively large size sorbent particles or beads (approximately 10 to 20 mesh) are relatively expensive to make, the adsorption rate is diffusion controlled and therefore relatively slow, the sorbent particles or beads are porous (note for example the micropores and macropores of the sorbent particles shown in FIG. 7 of the patents incorporated above by reference) and the center portions of such beads or particles tend not to be completely used in SO.sub.x and NO.sub.x adsorption, the relatively large beads with the relatively large mass tend to break and attrite when they impact a solid surface at a relatively high velocity, and since the apparatus used to practice the process illustrated in FIG. 1 is essentially a gravity feed process, the sorbent particles or beads upon adsorbing the SO.sub.x and NO.sub.x are at the top of the apparatus and the regenerated beads, due to the gravity flow utilized, are present at the bottom of the apparatus requiring that they be lifted back up to the top of the apparatus, such as by the pneumatic lift line 60 in FIG. 1, for recycling.
It has been found that sorbent particles or beads of a comparatively or relatively small size, approximately 70 mesh (210 microns) to 140 mesh (105 microns), have the following comparative advantages vis-a-vis the above-noted relatively large sorbent beads or particles. The smaller size sorbent beads or particles adsorb the SO.sub.x and NO.sub.x more rapidly because the process is chemically not diffusion controlled (see FIG. 2 where the NO.sub.x adsorption rate is compared for sorbent particles or beads of particle sizes 1,300, 630 and 300 microns, respectively; the removal rate for particles of 300 microns particle size is about 5 times greater than for the particles of 1,300 microns particle size), the relatively small size sorbent beads or particles are easier to make and therefore less expensive which in turn makes the SO.sub.x and NO.sub.x removal process less expensive, and it has been found or discovered that due to the relatively smaller mass of the relatively smaller sorbent beads or particles, the apparatus for lifting the regenerated beads to the top of the apparatus used to practice the NOXSO Process, such as the above-noted pneumatic lift line 60 in FIG. 1, can be utilized as both a vehicle for lifting the regenerated sorbent particles back up to the top of the apparatus and a vehicle for containing the sorbent particles or beads while they are adsorbing NO.sub.x and SO.sub.x from the flue gas.
Further, as known to those skilled in the fluid bed art and as noted above, solid particles such as the present sorbent particles or beads, have a fluidization velocity and a terminal velocity. The fluidization velocity is the velocity at which the sorbent particles or beads become fluidized, i.e. moves in a fluid state under the influence of a pressurized gas and the terminal velocity is the velocity at which the beads become entrained in the gas and then escape from the fluid state and from the fluid bed. While the above-noted relatively large sorbent beads or particles (approximately 10 to 20 mesh) due to their relatively large size and therefore relatively large mass have a relatively high terminal velocity and hence remain in the fluidized state when subjected, for example, to conditions in the fluid bed heater 18 and fluid bed cooler of FIG. 1 in the NOXSO Process, it has been found that the relatively small sorbent beads or particles (approximately 70 to 140 mesh) have a relatively low terminal velocity with the tendency to escape from the fluidized state and be lost when utilized in apparatus such as the fluid bed heater 18 and fluid bed cooler 52 of FIG. 1.
In view of the foregoing, it will be understood that there exists a need in the flue gas SO.sub.x and NO.sub.x removal art of new and improved process and apparatus which utilizes the above-noted relatively small sorbent beads or particles (approximately 70 to 140 mesh) and which permits the function of the pneumatic lift line 60 and fluid bed adsorber 70 of FIG. 1 to be combined in a single apparatus or process step, and further that there exists a need for new and improved apparatus and process for contacting the saturated sorbent particles with a heating gas to heat the saturated sorbent particles and remove the NO.sub.x therefrom and for contacting the heated sorbent particles with a cooling gas after NO.sub.x and SO.sub.x removal therefrom to cool the sorbent particles and produce cooled regenerated sorbent particles for continuous repeating of the NO.sub.x and SO.sub.x removal process.